The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.
Certain geological investigative operations involve drilling boreholes from which core samples are extracted. Analysis of material within the core samples provides geological information in relation to the underground environment from which the core sample has been extracted. Typically, it is necessary to have knowledge of the orientation of each core sample relative to the underground environment from which it has been extracted. For this purpose, it is usual to use an orientation device for providing an indication of the origination of the core sample.
Core drilling is typically conducted with a core drill fitted as a bottom end assembly to the bottom end of a series of drill rods. The core drill comprises an outer tube which is connected to the bottom end of the series of drill rods and an inner tube which is known as a core tube. A cutting head is attached to the outer tube so that rotational torque applied to the outer tube is transmitted to the cutting head. A core is generated during the drilling operation, with the core progressively extending into the core tube as drilling progresses. When the core tube is full or becomes blocked, the core tube is retrieved from within the drill hole, typically by way of a retrieval cable lowered down the drill rods. Once the core tube has been brought to ground surface, the core sample can be removed and subjected to the necessary analysis.
There are various proposals for attachment of the orientation device, or a downhole component thereof, to the core tube. One such proposal is disclosed in the applicant's international application WO 2006/024111, the contents of which are incorporated herein by way of reference.
The core tube and the orientation device, or a downhole component thereof, provides an assembly that is deployed within the outer tube. For this purpose, the assembly must descend within the drill rods to the outer tube, passing through fluid (such as drilling mud) contained within the drill rods. As the assembly descends, it is necessary for fluid within the drill rods to flow past the descending assembly. The fluid can easily flow through the core tube because of its construction, but the presence of the orientation device, or downhole component thereof, can provide an impediment to fluid flow. This can retard the rate of descent of the assembly, which can be undesirable as it prolongs the overall time required for the core sampling operation. Indeed, it is most desirable that the assembly be able to descend within the drill rods relatively rapidly so that time is not unnecessarily wasted during this stage the core sampling operation.
It is against this background, and the problems and difficulties associated therewith that the present invention has been developed.
While the background of the invention has been described in relation to deployment of a core sample orientation device, or a downhole component thereof, it should be understood that the invention may be applicable to deployment of any appropriate device within a borehole.